1. Field of the Invention
The present invention generally relates to an apparatus for use in a well. Particularly, the invention relates to an apparatus for use in forming a lateral wellbore. More particularly, the invention relates to an apparatus for forming a pilot ledge to begin the formation of a lateral wellbore.
2. Description of the Related Art
Multilateral systems enable multiple reservoirs or areas within a reservoir to be produced simultaneously and offer the opportunity for reduced drilling and completion costs, increased production, and more efficient reservoir drainage. Multilateral technology connects a lateral wellbore or multiple lateral wellbores to a main borehole at the multilateral junction. The multilateral junction can be designed in a new well application or created in an existing wellbore in a re-entry application. These advances in drilling technology have made many vertical drilled wells candidates for re-entry and re-work to drill lateral wellbores.
Starting a lateral in a cased wellbore requires forming a pilot ledge in the wellbore tubular to provide direction and a pathway for a bit to begin the drilling operation. Because most bits are designed to drill at their bottom end surface, the pilot ledge is formed in the wellbore to create a contact surface for the bottom of the bit to initialize continuous drilling and minimize reaming of the bore. The drilling of the lateral starts as the bottom portion of the bit contacts this pilot ledge and proceeds along a path determined by a concave portion of a whipstock.
A conventional method used to create a pilot ledge in a cased wellbore begins with the setting of a packer or a bridge plug at a depth below the intended window of the lateral. Thereafter, a starter mill connected to a whipstock by a shearable connection is run into the wellbore. The starter mill typically includes a mill with a nose portion. Blades are disposed on the outer surfaces of the mill for cutting the pilot ledge. The nose portion connects the starter mill to the whipstock. The whipstock is set or fixed at a certain orientation to provide a directional guide for the starter mill. With the whipstock anchored to the packer, a shearing force is applied to the run-in string to detach the starter mill from the whipstock. The starter mill is then raised and rotated and proceeds to work back down along a concave face of the whipstock. The whipstock directs the starter mill to the opposing wall of the wellbore to begin cutting the pilot ledge. When the desired pilot ledge is cut, the starter mill is retrieved and a window mill is run-in to form a window shaped opening in the casing for a tri-cone bit to subsequently drill the lateral wellbore.
A conventional method of starting a lateral in an open hole does not require a pilot ledge. Instead, a whipstock is set above an open hole bottom at a depth below the intended window of the lateral. Then, cement is supplied to fill the wellbore above the whipstock. Once cured, the cement provides a drillable medium for a standard drilling bit to initiate drilling. As the drilling continues, the bit is guided by the concave face of the whipstock to form the lateral.
The above described method is generally effective when applied to an open hole adjacent to relatively softer formations. However, problems arise when this method is applied to open holes adjacent to abrasive and hard formations such as sandstone and quartzite. One problem caused by these hard borehole walls is severe wear and tear on the concave face of the whipstock which comes about as a result of the cutting tool""s inability to penetrate the formation as it moves along the concave face of the whipstock. This problem is compounded by the fact that the sides of a bit generally are not designed to cut. Difficulty in cutting into the hard formation of the wellbore causes the bit to cut into the concave face of the whipstock. Consequently, the whipstock may have to be replaced before a lateral wellbore is formed.
One solution to the problem of hard borehole walls is to form a pilot ledge using the conventional method for a cased wellbore. However, the use of a starter mill presents the same problems, most notably, severe wear and tear on the whipstock as a result of the cutting tool""s inability to penetrate the hard formation as the starter mill moves along the whipstock.
In addition to wear and tear, binding problems can also occur when conventional methods of forming a pilot ledge are applied to an open hole with hard formations therearound. Generally, a starting mill should have the same profile as a window mill or a bit that forms the lateral wellbore in order to leave an adequate clearance for the cutting tools that follow. When the formation is hard, continuous drilling may alter the profile of the starter mill. As a result, the pilot ledge formed will have a smaller diameter than the bit that follows. With its larger profile, the bit will bind and be forced to ream the pilot ledge to create a proper profile for itself. In the process, the bit may be damaged and its profile altered resulting in a wellbore that is not accessible by other tools.
Therefore, there is a need for an apparatus and methods to more effectively form lateral wellbores from hard, open-hole primary wellbores. There is a further need for an apparatus that can efficiently form a pilot ledge in a wellbore. There is yet a further need for a tool than can efficiently form a pilot ledge in an open hole wellbore adjacent a hard, abrasive formation.
The present invention generally provides an apparatus and method for forming a pilot hole in a formation. In one aspect of the present invention, the apparatus may comprise a starter mill connected to a bearing mill by a body joint. The apparatus may further comprise a lead bearing connected to a starter mill by a lead joint. Preferably, an outer diameter of the bearing mill is about the same as an inner diameter of a wellbore. As the lead bearing travels along the concave, the apparatus will bend between the bearing mill and the lead bearing. The bend urges the starter mill into contact with the wellbore wall.
In another aspect of the present invention, a method for forming a pilot hole in a wellbore includes running a tool into the wellbore, the tool comprising a starter mill disposed between a first bearing and a second bearing. While running the tool along a concave of a whipstock, the tool bends between the first and second bearing and urges the starter mill to form the pilot hole.